1. Field of the Invention
The present invention generally relates to a proximity IC card (hereunder abbreviated to xe2x80x9cPICCxe2x80x9d) and, more particularly, to a detection circuit for use in a PICC reader/writer (hereunder abbreviated as PICC-R/W) for writing data to and.reading data from a PICC.
2. Description of the Related Art
PICC standards are described in ISO/IEC (International Organization for Standardization/International Electrotechnical Commission) 14443. Hereinafter, in relation to the present invention, a brief description is given about a part of ISO/IEC 14443 which relates to a type-B communication interface for a PICC and which describes the properties and characteristics of a field that provides power transmission and two-way or bidirectional communication between a PICC and a proximity coupling device (hereunder abbreviated to PCD), such as the PICC-R/W.
(1) Power Transmission from PCD to PICC
To supply effective power to the PICC in a radio frequency (RF) operating field, a carrier (having a carrier frequency of fc=13.56 MHz)is transmitted from the PCD to the PICC, whereupon the received carrier is rectified to thereby generate electric power needed for an operation of an internal circuit.
(2) Communication from PCD to PICC
The PCD transmits data to the PICC by amplitude-shift-keying (ASK) modulating the amplitude of the carrier with a modulation index or factor of 10% at a data bit rate of 106 Kbps (=fc/128).
(3) Communication from PICC to PCD
The PICC transmits data to the PCD by performing load modulation of a load for reception of the carrier at a frequency fs(=fc/16), which is ({fraction (1/16)})the carrier frequency, to thereby generate a subcarrier (whose frequency fs=847 kHz), and by then binary-phase-shift-keying (BPSK) modulating the phase of the subcarrier at a data bit rate of 106 Kbps (=fc/128).
FIG. 1 schematically shows the constitution of an example of a PICC.
In the case 6f the example of FIG. 1, two chips respectively constituting a central processing unit (CPU) portion 11 and an RF portion 12 are incorporated into a card body 10. Further, an antenna (AT) 13, wound like a coil, is disposed along the periphery of the card body 10. The CPU portion 11 is constituted by what is called a one-chip computer and includes a CPU, memories such as a ROM, a RAM, and an EEPROM, and an input/output (I/O) interface.
FIG. 2 shows the constitution of an example of the communication interface between a PCD and a PICC.
In the case of the communication from the PCD to the PICC, which has been described in the foregoing section (2), a modulation portion (MOD) 20 of the PCD performs ASK modulation of the amplitude of a carrier (having a carrier frequency of fc=13.56 MHz) with a modulation index of 10%. Then, a resultant.signal is transmitted from the PCD to the PICC through output amplifiers 22 and 23 and an antenna 24.
In contrast, in the case of the communication from the PICC to the PCD, which has been described in the foregoing section (3), a load 26 for reception of an RF signal is varied under the control of a modulation portion (MOD), 28 of the RF portion 12 of the PICC shown in FIG. 1. Then, a BPSK modulation for providing binary phase information (representing 0 or 180 degrees) is performed on a subcarrier (whose frequency fs=847 kHz) generated by a load modulation (resulting in an amplitude modulation (hereunder referred to as an AM modulation)).
The modulated signal is transmitted to the PCD through an antenna 25 (corresponding to the antenna 13). Actually, a detection portion (DET) 21 of the PCD detects the carrier that is outputted by the PCD itself and that undergoes the load modulation (including the BPSK modulation) performed by the PICC.
FIG. 3 shows the constitution of an example of the conventional detection portion 21.
In the communication interface of FIG. 2, a signal received by the antenna 24 is amplified by an amplifier circuit 1 comprising a transistor (Tr1) 32. Subsequently, the amplified signal undergoes a half-wave rectification performed by a diode (D1) 34 and a capacitor 35 of the next stage that is a detection circuit. In the case of such a half-wave rectified signal, the subcarrier signal having undergone the AM modulation (including the BPSK modulation) performed by the PICC is superposed onto a direct current (DC) component thereof. Then, such a half-wave rectified signal is amplified to a predetermined level in the next stage that is an amplifier circuit 2 comprising a transistor (TR2) 37.
As described above, in the circuit of the conventional detection portion, usually, the subcarrier undergoing the AM modulation(including the BPSK modulation) is detected by the diode 34. Moreover, the level of the received signal largely varies with the distance between the PCD and the PICC. It is, therefore, necessary, for receiving and detecting a micropower subcarrier, to amplify a carrier itself including a subcarrier in the amplifier circuit 1 of the first stage so that the amplified voltage level is not less than the forward voltage (Vf) of the diode 34.
Furthermore, in addition to the aforementioned amplification of the carrier by the amplifier circuit 1, it is necessary, for limiting the level of a detected signal to within a receivable signal level range by simultaneously avoiding signal saturation, to amplify the carrier and the detected signal, whose level has dropped by the forward voltage of the diode 34 after the detection thereof, in the amplifier circuit 2 of the next stage. As a result, the conventional detection portion has complex constitution and a large number of components.
Additionally, the conventional detection portion has a drawback in that waveform distortion occurs in the amplified signal because the amplifier circuit 1 of the first stage amplifies the micropower signal by using a non-linear region thereof. Further, in the conventional detection portion of the circuit constitution of FIG. 3, bias points of the amplifier circuits 1 and 2 are determined according to the level of the received signal and vary with the distance between the PCD and the PICC. Thus, the conventional detection portion has another drawback in that it is difficult to determine the amplification factors of the amplifier circuits 1 and 2 in the case of comprehensively judging the aforementioned conditions such as the forward voltage drop across the diode 34, the saturation level of the amplified signal, and the range of the receivable signal level.
Accordingly, in view of the aforementioned drawbacks of the conventional detection circuit, an object of the present invention is to provide a detection circuit which can easily determine the amplification factors of the amplifiers, and which can reduce the waveform distortion of an amplified signal, and which has a simple structure comprising a smaller number of components.
To achieve the foregoing object, according to an aspect of the present invention, there is provided a detection circuit for detecting a subcarrier signal sent from a PICC and superposed onto a carrier signal received through an antenna. This detection circuit comprises a bias circuit for applying a predetermined DC potential to a signal received from the antenna, a rectifier circuit for extracting a subcarrier signal superposed onto the carrier signal by rectifying a signal received from the antenna at the DC bias point, and an amplifier circuit for amplifying the subcarrier signal extracted at the bias point.
This rectifier circuit comprises a transistor circuit having a base terminal to which a bleeder voltage is supplied, from a power supply bleeder circuit as a DC bias, together with the signal received from the antenna. The transistor circuit further has a collector terminal connected to a power supply, and has an emitter terminal to which a resistor and a capacitor are connected. A rectified subcarrier signal is outputted from the emitter terminal.
Further, according to another aspect of the present invention, there is provided a detection circuit for detecting a data signal sent from a PICC and superposed onto a carrier signal received through an antenna. This detection circuit comprises a bias circuit for applying a predetermined DC potential to a signal received from the antenna, a rectifier circuit for extracting a data signal superposed onto the carrier signal by rectifying a signal received from the antenna at the DC bias point, and an amplifier circuit for amplifying the data signal extracted at the bias point.
The bias circuit is a power supply bleeder circuit for dividing a power supply voltage. Further, the rectifier comprises a transistor circuit having a base terminal to which a bleeder voltage is supplied, from the power supply bleeder circuit as a DC bias, together with the signal received from the antenna. The transistor circuit further has a collector terminal connected to a power supply and has an emitter terminal to which a resistor and a capacitor are connected. A rectified data signal is outputted from the emitter terminal.